"NiMH dislikes overcharge, and the trickle charge is set to around 0.05C. NiCd is better at absorbing overcharge and the original NiCd chargers had a trickle charge of 0.1C. The differences in trickle charge current and the need for more sensitive full-charge detection render the original NiCd charger unsuitable for NiMH batteries. A NiMH in a NiCd charger would overheat, but a NiCd in a NiMH charger functions well. Modern chargers accommodate both battery systems.
It is difficult, if not impossible, to slow charge a NiMH battery. At a C rate of 0.1C to 0.3C, the voltage and temperature profiles do not exhibit defined characteristics to trigger full-charge detection, and the charger must depend on a
timer. Harmful overcharge can occur when charging partially or fully charged batteries, even if the battery remains cold. "
https://batteryuniversity.com/learn/article/charging_nickel_metal_hydrideSo if you have unmatched batteries, you can charge it at 0.05 C for a long time in a string, I think, if you cannot get sense lines on them. I think thats OK, I like NIMH too for things like lab amplifiers and weird shit, don't feel like dealing with lithium there.. this is easier. But I just take them out of the circuit and charge them on a battery charger, instead of charging in circuit. 20S is a little bit hard core for D cells.
Here is a 'field report' about what you want to do. I personally have to charge 9X d Cells and 8X C cells and I don't wanna take the screws out of my battery boxes, so I am doing research.
https://www.candlepowerforums.com/vb/showthread.php?239572-Charging-12-NiMh-Batteries-in-SeriesI share your belief that all the people saying you should integrate charge controllers and other stuff into simple projects are pretty crazy (every battery thread ever would have you double or triple costs to make it plug into something). I recommend investing in quality battery packs that don't break with removing cells lol.
I am trying 1/20C on a string of 10 D cells right now.
Here is some real info, its not THAT scary like people make it out, panasonic lists it as an option
https://www.epectec.com/downloads/Panasonic-NiMH-Cell-Lineup.pdfMy guess is that you will get like, slightly more life time out of the batteries you go all out to keep charged (10% ? ) vs the ones you series slow charge. For me that is good enough.